Got to B flight connections at 4.55am.....and there was no one to process us until 5.15??
Not unusual! Security starts at 05:00 hrs, and staff are allowed extra time to get to B from A. However the HKG service is in the habit of arriving early. Still, I'm sure you were through in short order.
Incidentally, had you gone to T5A instead, you may have had to wait there, sometimes it's the BA staff who don't turn up until 05:00 hrs or a little after.